Conventionally, a power source device of this kind is disclosed in Japanese Patent Application Unexamined Publication No. H10-295086. FIG. 9 is a circuit diagram illustrating a configuration of a conventional power source device. As shown in FIG. 9, transistors 3 and 4 are connected to DC power source 2 via input terminal 1, and output terminal 7 and load 8 such as a roller of a printer are connected to a connecting node between transistors 3 and 4 via transformer 5 and low-pass filter 6.
Next, an operation of a conventional power source device configured as mentioned above is described. FIG. 10 shows ideal voltage waveforms to illustrate an operation of a conventional power source device. Voltage waveform G of output terminal 7 as shown in FIG. 10 is detected by voltage detecting circuit 9 and input into a negative input end of comparator 10, and an ideal alternating output voltage signal from alternating signal generator 11 is input into a positive input end of comparator 1, thereby carrying out comparison. Then, comparator 10 can obtain an output voltage having ideal pulse-like voltage waveform H as shown in FIG. 10. By inputting this output voltage into a base of transistors 3 and 4, a voltage of DC power source 2 is made into a pulse form, and the feed-back voltage is input into transformer 5.
Such a conventional power source device has had a problem of a switching loss in a switching element. That is to say, in a power source device having a conventional configuration, a voltage is actually applied to each input end of comparator 10 in a way in which a noise voltage such as a ripple voltage is superimposed in addition to an ideal voltage waveform from voltage detecting circuit 9 and alternating signal generator 11. As a result, ideal pulse-like voltage waveform H shown in FIG. 10 may be changed.
This situation is described in detail with reference to FIG. 11. FIG. 11 is an enlarged view showing a voltage waveform to illustrate an operation of a conventional power source device. FIG. 11 is an enlarged view showing time interval “d” shown in FIG. 10. Although comparator 10 should obtain an output voltage of ideal pulse-like voltage waveform J as shown in FIG. 11, it actually obtains voltage waveform K having a large number of pulse-like waveforms in time interval “d”. Then, voltage waveform K is input into the base of switching elements 3 and 4. As a result, switching elements 3 and 4 carry out unnecessary switching operations. For example, transistor 3 carries out switching operations at times corresponding to the number of logical values high of voltage waveform K shown in FIG. 11, and transistor 4 carries out switching operations at times corresponding to the number of logical values low thereof. Accordingly, a switching loss is increased.